This invention relates to a transmission power control method and transmission power control device in a CDMA mobile communication system. In particular, this invention relates to a transmission power control method and transmission power control device which control the uplink transmission power of a mobile station such that a target SIR is obtained for signals received from the mobile station on the mobile network side, and which also control the target SIR based on the quality of the transmission path.
(1) Transmission Power Control of the Prior Art
In mobile communication systems conforming to 3GPP specifications, transmission power control is executed such that prescribed error rates are obtained on the network side and on the mobile station side, and such that the transmission power does not become excessive. FIG. 12 explains transmission power control, and shows a case in which the mobile station uplink transmission power is controlled on the network side.
Signals transmitted from a mobile station 1 are demodulated by the demodulator 2a of the base station 2, and then decoded by the error-correcting decoder 2b. Then CRC error detection is performed for each transport block TrBk in the CRC detector 2c. The error detection result for a transport block TrBk is conveyed to the target SIR control portion 3a of the base station control device 3. The target SIR control portion 3a reduces the target SIR if there are no errors in a prescribed observation interval, and if there are errors, increases the target SIR based on the block error rate, and sets the target SIR in the storage portion 2d of the base station 2. The comparator 2e compares the target SIR with the measured SIR of received signals, measured by the SIR measurement portion 2f, and based on the comparison result creates a TPC bit to control the transmission power of the mobile station 1, and transmits the TPC bit to the mobile station 1. That is, if the measured SIR is greater than the target SIR, a TPC bit is created so as to lower the transmission power by a fixed amount, and if the measured SIR is less than or equal to the target SIR, a TPC bit is created so as to raise the transmission power by a fixed amount; the TPC bit is then transmitted to the mobile station over a dedicated physical control channel DPCCH. The mobile station 1 controls its own transmission power according to the received TPC bit. The feedback loop ILP which controls the uplink transmission power of the mobile station is called the inner loop; the base station 2 performs inner loop transmission power control. The feedback loop OLP which controls the target SIR is called the outer loop; the base station control device performs outer loop transmission power control.
In uplink outer loop transmission power control, when the base station 2 transmits data received from the mobile station 1 to the base station control device 3, information on the quality over the wireless interval (between the mobile station and base station) is appended to the data. Here, quality information are parameters added to the Iub frame protocol in the 3GPP system, and are (1) QE (Quality Estimates), which are error correction information (for example, error correction rates) over the wireless interval, and (2) a CRCI (CRC Indicator), which is the result of judgment of the CRC appended to each transport block TrBk. The base station control device 3 measures the quality of received data over a fixed length of time based on this quality information.
As stated above, quality information includes CRCI and QE; when using CRCI, the base station control device 3 computes the BLER (block error rate) from the CRCI, and uses a comparison of the BLER representing the measured quality and a BLER given as the target quality to compute the uplink target SIR used in inner loop transmission power control. When using QE, the base station control device 3 converts the QE into a BER (bit error rate), computes the average value of the BER over a measurement interval, and uses a comparison of the BER representing the measured quality and a BER given as the target quality to compute the uplink target SIR used in uplink inner loop transmission power control. Computation of the target SIR may be performed in units of received frames, or a measurement interval may be set and computation performed after the measurement interval has expired. If the target SIR obtained in this way differs from the previous value at the end of the measurement time, the new target SIR is input to the base station 2.
(2) Transmission Power Control During Handover
The above is a case in which one mobile station communicates with one base station; during handover, as shown in FIG. 13, the mobile station 1 communicates simultaneously with two base stations 2, 2′. In this case, the base station control device 3 employs the data selection portion 3b to select the uplink data received from the plurality of base stations 2, 2′, as shown in FIG. 14, the data with better quality (for example, the data with better quality according to comparison of QE, CRCI, or similar), and the target SIR control portion 3a (FIG. 12) performs quality measurement of the selected data and determines the target SIR. Selection of the data with better quality is called selective diversity, and control at the time of handover is called DHO (Diversity Hand Over). Because quality is improved as a result of selective diversity, the gain resulting from selective diversity is called the diversity gain. In the case of a connection with a single base station, there does not exist other selectable data, selective diversity is not possible, and so diversity gain is not obtained.
The base station control device 3 notifies all base stations 2, 2′ for which a connection exists with the mobile station 1 shown in FIG. 14 of the target SIR (Eb/No) computed based on the above data after selection, in conformance with the interface Iub between the base station and the base station control device, and each base station performs inner loop transmission power control based on the target SIR thus received.
(3) Problems
During handover, the DHO (Diversity Hand Over) control explained in FIG. 14 is performed. Hence a target SIR computed for the quality obtained by the diversity gain due to selective diversity is set simultaneously in all base stations, and all base stations perform inner loop transmission power control based on this same value for the target SIR (see FIG. 15). Consequently, in conventional outer loop transmission power control, there is the problem that quality states for each of the transmission paths are not taken into consideration in handover states having a plurality of transmission paths to base stations. In FIG. 15, the polygonal line ZZL is the measured SIR, and the dashed line DL is the target SIR. Uplink transmission power control is performed based on the magnitude relation between the measured SIR and target SIR, and the measured SIR changes, as shown by the polygonal line, as a result. The target SIR is updated in each measurement interval.
Specific problems are explained below, but the following points (1) through (4) must be taken into consideration.
(1) The quality obtained by the diversity gain of selective diversity is better, in the DHO state, than the quality of data transmitted from individual base stations. (2) Further, in conventional methods, upon entering the DHO state (at the start of handover), the uplink target SIR values set in base stations 2, 2′ may not be the same for all base stations. (3) In addition, the result obtained may not necessarily indicate that the uplink target SIR in inner loop transmission power control and the quality of data actually received (or the reception SIR) are equal. (4) Further, due to the nature of outer loop control, because the update interval of the target SIR used in inner loop control is extremely long compared with inner loop control, insofar as possible an appropriate target SIR value must be set. If an inappropriate target SIR value were set, transmission power control will be performed according to this inappropriate update value until the time of the next update.
Non-DHO State/Initial DHO State
When the mobile station 1 and the base station 2 are communicating one-to-one, the uplink target SIR value sent to the base station 2 (FIG. 13) and the reference SIR in uplink outer loop transmission power control coincide (see FIG. 16). At this time, the DHO state has not yet been entered, and so there is no need to consider diversity gain due to selective diversity. Hence due to uplink outer loop transmission power control, the updated SIR value (the target SIR value after updating) may be larger than, equal to, or smaller than the reference SIR (the uplink target SIR of the base station 2), depending on the reception quality (see FIG. 16).
Thereafter, upon entering the DHO state, as shown in FIG. 17, the uplink target SIR for inner loop transmission power control set in the newly added base station 2′ may be different from that in the base station 2. This is because the conditions of the wireless interval differ for the base stations 2 and 2′, so that the values need not be the same. After entering this DHP state, when outer loop transmission power control is executed, the updated SIR is computed for the quality obtained through the diversity gain of selective diversity, as explained above. At this time, the uplink target-SIR value set in base station 2 is used as reference, and so the uplink SIR value set in base station 2′ is not taken into consideration (FIG. 17).
Normally the reference SIR value used in outer loop transmission power control must be used to determine the updated SIR based on the virtual SIR due to the DHO effect, taking into account the value set in the base station 2′, as shown in FIG. 18. The above is a problem in the initial DHO state.
Problems in the DHO State
Quality judgments performed in outer loop transmission power control are determined by received data error rates and other quality information. However, the average reception level over a short time used in transmission power control does not necessarily correspond to the received data error rate. This is because the received data error rate is not determined only by the reception level, but is also affected by the period of fading and delay profile conditions. Hence the uplink target SIR value set in the newly added base station during DHO cannot be easily inferred from the reception quality; that is, cases are possible in which the average reception level is high and the SIR value is high even when quality is poor. Below, examples of the problem are explained in light of these considerations.
(a) Example of a case in which quality of different transmission paths is poor, but with selective diversity the target quality or better is obtained:
As shown in FIG. 19, there are cases in which the quality of individual transmission paths is worse than the quality target value, but with selective diversity, quality equal to or exceeding the target quality is obtained. That is, even if the quality for the base-stations 2, 2′ (indicated by black circles) is below the target quality (BER=1/100 or above), upon entering the DHO state, the quality resulting from selective diversity (black circle) may be equal to or above the target quality (BER less than 1/100). In this case, the initially calculated updated SIR value is smaller than the uplink target SIR value at the current base station 2. With respect to base station 2′, if the target SIR value were lower than the target value set in the current base station 2, and also lower than the updated SIR value, then rather than lowering the SIR values of the base stations 2, 2′, an operation is instead performed to unnecessarily raise the target SIR value for the base station 2′.
By this means the transmission path quality for base station 2′ is improved, and there is the possibility that the quality with selective diversity may be excess quality. And in communications with other mobile stations (in particular, mobile stations served by the base station 2′ and similar), noise, that is interference, will be increased, possibly leading to degraded quality.
(b) Example of a case in which quality varies for different transmission-paths, and quality is poor even with diversity gain:
As shown in FIG. 20, there are cases in which the quality of each transmission path is poor, and the quality with selective diversity is also lower than the target quality. That is, there are cases in which the quality (black circles) of the base stations 2, 2′ is lower than the target quality (BER=1/100 or higher), and when the DHO state is entered, the quality (black circle) due to selective diversity is also lower than the target quality (BER=1/100 or higher). In such cases, upon entering the DHO state, the initially calculated updated SIR value is higher than the uplink target SIR value of the current base station 2. With respect to base station 2′, if the uplink target SIR value is higher than the value set for the current base station 2, and is also higher than the updated SIR value, then rather than raising the separate SIR values, an operation is instead performed to lower the value for the base station 2′. As a result there is the possibility that the transmission quality for the base station 2′ is worsened, an appropriate diversity gain cannot be obtained, and the quality with selective diversity is worsened.
(c) Excessive Increase in SIR Value
As shown in FIG. 21, there are cases in which the quality of each transmission path is poor, and the quality with selective diversity is also below the target quality. That is, there are cases in which the quality (black circles) of the base stations 2, 2′ is lower than the target quality (BER=1/100 or higher), and when the DHO state is entered, the quality (black circle) due to selective diversity is also lower than the target quality (BER=1/100 or higher). Suppose that in such a case, cause of poor reception quality, the SIR value of base station 2 is the highest value for obtaining the required target quality through power control up to immediately before the DHO state. If at this time the uplink target SIR value set in the newly added base station 2′ is low, and moreover data quality is poor, the updated SIR value will be higher than the value set in base station 2. Consequently the amount of change in the target SIR value at base station 2′ is be extremely large, prompting an increase in power, and power control will result which intensifies interference with other mobile stations.